Component mounted to substrate with overlying bridge-shaped supporte

ABSTRACT

An optical component such as a laser chip is mounted on a substrate. The chip is mounted on a bridge positioned on the substrate locator in the form of depending legs provided on the bridge so that the bridge is located in the vertical direction relatively to the substrate and hence the laser chip is also located. Finally, the chip is secured to the substrate by, for example, soldering.

This is a division of application Ser. No. 07/080,564, filed June 11,1987, now U.S. Pat. No. 4,846,930.

BACKGROUND OF THE INVENTION

The invention relates to a method of mounting a component to asubstrate, for example the mounting of an optical component such as alaser chip to a substrate.

Recent developments in optical technology have led to the constructionof laser chips and photosensor chips which have relatively smalldimensions of the order of 200 microns. It is now proposed that thesecomponents should be mounted on substrates and accurately aligned withoptical waveguides or other optical components. One of the difficultieswith this is that it is difficult to hold the component accurately usinga micromanipulator or the like during mounting of the component on asubstrate.

A paper by M. Kobayashi et al entitled "Guided-Wave optical gate matrixswitch" in the Proc. 11th European Conference on Optical Communication(pages 73-76) describes the mounting of a laser diode to a silicon heatsink. The heat sink is a slab of silicon which is apparently laid on thesubstrate. The laser diode cannot, however, be accurately positioned onthe substrate.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method ofmounting a component on a substrate comprises mounting the component ona support; positioning the support on the substrate, wherein at leastone of the support and the substrate includes locator means such thatthe support is located in at least one direction relatively to thesubstrate; and securing the component to the substrate.

The location of the support in at least one direction relatively to thesubstrate automatically locates the component also in that direction.Thus, the locator means can be positioned remotely of the component andwill not interfere with the mounting of the component.

In accordance with a second aspect of the present invention we providein combination a component secured to a substrate, the combinationfurther comprising a support to which the component is mounted, at leastone of the support and the substrate including locator means forlocating the support in at least one direction relatively to thesubstrate.

In one simple arrangement, the locator means comprises two or more legspositioned on either the support or the substrate, the support beingpositioned on the legs spaced from the substrate by a predeterminedamount determined by the length of the legs.

Preferably, however, complementary locating portions are formed in thesupport and the substrate. This has the advantage that the support islocated in two directions relative to the substrate.

For example, the locating portions may comprise complementary ridges andrecesses. Where the substrate comprises a single crystal such as siliconit is particularly convenient if the complementary ridges and recesseshave a V-shaped cross-section since these can be formed using knownmasking and anisotropic etching techniques.

The component may be directly bonded to the substrate, for example bysoldering, or indirectly by bonding the support to the substrate.Preferably, both the support and component are bonded to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Some examples of methods and combinations in accordance with theinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a side elevation of a first example with some parts omittedfor clarity;

FIG. 2 is a plan of the first example with some parts omitted forclarity; and,

FIGS. 3, 4 and 5 are a side elevation, partial end elevation, and planrespectively of a second example.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a lithium niobate substrate 1. Titanium isdiffused into a narrow rectilinear section 2 of the top surface of thesubstrate 1 to define an optical waveguide by modifying the refractiveindex of the substrate. A generally U-shaped recess or slot 3 is thencut in the surface of the substrate 1 orthogonal to the waveguide 2 byusing a suitable technique such as ion beam milling or reactive ionetch. As can be seen in FIG. 1, the slot 3 has a rectangularcross-section. It will be seen that the formation of the slot 3 dividesthe optical waveguide 2 into two subsidiary optical waveguides 5, 6which are automatically in alignment with one another. A conventionallaser chip 4 is then mounted in the slot 3 (by a method to be describedbelow) with opposite facets 7, 8 in alignment with the subsidiarywaveguides 6, 5 respectively.

The depth (D) of the slot 3 is chosen so that the light emitting stripein the laser chip 4 is matched to the optical waveguides 5, 6. Thelength (L) of the slot 3 is chosen to allow the maximum possible lighttransmission between the laser chip 4 and the substrate 1 and it can betailored to suit a given chip length. The third dimension (W) is notcritical and is chosen to allow adjustment of the laser chip 4 along theslot 3 to enable the optimum position of the laser chip relatively tothe optical waveguides 5, 6 to be found and to permit a number of chipsto be mounted side by side.

Typically, L is about 200 μm and D<15 μm.

To mount the laser chip 4, the laser chip is initially soldered to theundersurface of a metal bridge 9. The bridge 9 has a pair of integraldepending legs 10 which rest on an upper surface of the substrate 1 withthe laser chip 4 suspended in the slot 3. This locates the laser chip 4in the vertical direction by controlling the extent to which the laserchip 4 is received in the slot 3. Furthermore, it is easier for amicromanipulator to hold the bridge 9 than the laser chip itself.Optimum alignment of the laser chip 4 with the optical waveguides 5, 6is achieved using an optical method by monitoring the power transmittedalong the waveguides while the activated laser chip is moved along theslot 3. Once the optimum position has been found, corresponding tomaximum power coupling with the waveguides 5, 6, the laser chip 4 andthe legs 10 of the bridge 9 are soldered to the substrate 1.

There are a number of advantages in providing two optical waveguides 5,6. In general, the spectral performance of conventional laser chipsneeds to be improved and this can be achieved by monitoring the laseroutput from the facet 8 while the main laser output is generated fromthe facet 7. In addition, this access to both facets could be used in acombined transmitter/receiver or simply to monitor the output power.

A transverse lower connection 11 to the laser chip 4 extends along thebase of the slot 3 (FIG. 2).

One of the advantages of providing an elongate slot 3 is that a numberof laser chips could be mounted side by side. This is shown in FIG. 2where additional laser chips 12, 13 are provided in alignment withoptical waveguides 14, 15, 16, 17 respectively, each pair having asimilar form to the optical waveguides 5, 6. In FIG. 2, bridgescorresponding to the bridge 9 and supporting the laser chips 12, 13 havebeen omitted.

The advantage of lithium niobate is that it can be used to formelectro-optic components which would be incorporated into areas of thesubstrate adjacent the slot 3 with suitable connections being made withthe optical waveguides.

FIGS. 3 to 5 illustrate a second example in which a silicon substrate 18is used. One of the advantages of silicon is that it can be veryaccurately etched using its anisotropic etching properties to producegrooves with depths accurate to 1 micron and with accurately determinedincluded angles. In the example shown in FIGS. 3 to 5, initially a flatbottomed channel 19 is formed having a generally U-shaped cross-sectionwith sloping sides by etchihg the 111 faces of the crystal. Subsequentlya pair of parallel V-shaped grooves 20 are etched parallel with thechannel 19 and on either side of the channel 19. A V-shaped groove 21 isetched at right angles to the channel 19 and grooves 20, having a depthapproximately equal to half the diameter of a monomode optical fibre 22which is subsequently to be mounted in the groove.

A photodiode 23 is bonded (eg. soldered) to the sloping surface of thechannel 19 facing the optical fibre 22.

A second silicon substrate or chip 24 is provided which corresponds tothe bridge 9 in the previous example. The chip 24 has two pairs ofdepending, V-shaped ridges 25 and a central depending ridge 26. Theincluded angle of each ridge 25 is substantially the same as theincluded angle of the V-shaped grooves 20 in the substrate 18.

A laser diode 27 is bonded to the ridge 26.

The substrate 24 is then mounted on the substrate 18 with each pair ofridges 25 being received in the corresponding groove 20 and straddlingthe groove 21. The depth of the grooves 20 and the height of the ridge26 are chosen such that when the substrate 24 is mounted on thesubstrate 18, the laser diode 27 is accurately located and aligned withan optical fibre 22 in the groove 21 (FIG. 4).

The provision of the grooves 20 and complementary ridges 25 assists inaccurately positioning the laser diode 27 in two directions and thisshould be contrasted with the previous example in which the bridge 9permits a certain degree of movement transverse to the slot 3.

A feature of this example is that the position of the laser diode 27with respect to the optical fibre 22 can be adjusted in the direction ofthe grooves 20 to obtain maximum power coupling into the optical fibre.In addition, the separation of the end of the optical fibre 22 from thelaser diode 27 can also be altered by sliding the fibre within thegroove 21. Once the correct relative positions have been found, theupper and lower silicon substrates 18, 24 are bonded together in such away that the laser diode attachment to the upper silicon chip 24 isunaffected. In addition, the optical fibre 22 is bonded into the groove21. Bonding may be achieved using soldering or any other knowntechnique.

The photodiode 23 may be used for a variety of purposes similar to thoseoutlined in the previous example for monitoring laser emission from thefacet of the laser diode 27 opposite to the optical fibre 22.

In a modification of this example (not shown) the upper substrate 24 mayinclude a further depending ridge which clamps the optical fibre 22 intothe groove 21.

I claim:
 1. In combination:a component secured to the lower side of abridge-shaped support, at least one of the support and the substrateincluding locator means for locating the support in at least onedirection relatively to the substrate, said support and component beingsecured to a substrate with the component located between the supportand the substrate.
 2. A combination according to claim 1, wherein thelocator means locates the support in two directions relatively to thesubstrate.
 3. A combination according to claim 2, wherein the supportand the substrate have complementary locating positions constituting thelocator means.
 4. A combination according to claim 3, wherein thelocating portions comprise complementary ridges and recesses.
 5. Acombination according to claim 4, wherein the locating portions comprisecomplementary V-shaped ridges and V-shaped recesses.
 6. A combinationaccording to any of claims 2 to 5, wherein the component comprises anoptical component.
 7. A component according to claim 6, wherein thecomponent comprises a laser chip or an optical sensor chip.
 8. Acombination according to any of claims 2 to 5, wherein one or both ofthe substrate and support comprise a single crystal.
 9. A combinationaccording to claim 8, wherein one or both the substrate and supportcomprise silicon.
 10. A combination according to any of claims 2 to 5,wherein the support is a bridge-like structure.